A Bank of Japan Policy Board member on Thursday issued a warning on downside risks for the nation's economy, citing the possibility of slower-than-expected growth in exports and dwindling personal consumption.

Sayuri Shirai said she was "uncertain" whether the central bank would be able to achieve its goal of 2 percent artificial inflation in fiscal 2015, and of the time frame required to shift to a growth path that can sustain that rate of inflation.

"I maintain the view that it will likely take longer than two years to achieve the 2 percent inflation — in order to avoid imposing excessive burdens on firms and households," she said in her speech in Naha, Okinawa Prefecture, arguing that the effects of a further consumption tax hike planned in October 2015 should also be examined.

Shirai is one of three policymakers who opposed the bank's outlook report released on April 30, which projected its 2 percent inflation target would be achieved in 2015.